Spin Transfer and Magnetic Interaction via Phosphorus in Nitronyl Nitroxide Radical-Substituted Triphenylphosphine Derivatives

Abstract

The magnetic behavior of polyradicals is usually understood on the basis of the spin polarization scheme displayed by the molecules. As far as conjugated hydrocarbon species are concerned, the spin distribution is rather well established and follows the sign alternation principle. Yet when a heteroatom is involved in the exchange pathway, the situation may become more delicate. This study concerns the involvement of a P-atom in the spin distribution and the intramolecular exchange interaction of nitronyl nitroxide radical-substituted triphenyl phosphine derivatives. The spin distribution of mono-, bi-, and tri-radical phosphine derivatives has been investigated by high-resolution fluid solution EPR and 1 H and 31 P MAS NMR spectroscopy. These techniques permitted one to establish that spin density is located at the phosphorus atom and showed that its sign depends on whether there is a P-lone pair or not. They also revealed the spin polarization scheme for the molecules. While these schemes are in line with ferromagnetic interaction between the radical units, the actual interactions are extremely weak as found by the magnetic studies. The experimental data are supported by DFT computations (UB3LYP/Lanl2DZ), which reproduce very well the effective spin distribution on these molecular systems except the sign inversion observed when going from the phosphine to its phosphine oxide counterpart. They also predict a very small high-spin/low-spin gap for these polyradicals.

Discrepancy between the Spin Distribution and the Magnetic Ground State for a Triaminoxyl Substituted Triphenylphosphine Oxide Derivative

“…It can be noticed that the coupling constant a P found by EPR for compound 1 is an order of magnitude larger than the one determined for the related nitronyl nitroxide triradical derivative. [14] The same trend is observed by 31 P NMR for the molecule which exhibits a resonance at À1781 ppm, confirming that more spin is located on P for 1 than for the corresponding nitronyl nitroxide substituted triradical. This increased spin density can be attributed to the aminoxyl radical involved here but also to the fact that for compound 1 the phosphorus is linked to phenyl C atoms bearing positive spin, that is, atoms with a higher degree of spin while for the related nitronyl nitroxide derivative the phosphorus is linked to C atoms with smaller negative spin densities.…”

“…Such a line broadening due to dipolar interactions, when going from a mono-radical to a triradical derivative, was observed for related series of polyradical compounds. [14] The high-resolution spectrum obtained for triradical 1 in fluid solution suggests that the spin density is spread over the core of the molecule. Moreover, the observation of a coupling with the P clearly indicates that spin density is located on this atom.…”

“…This has been confirmed by using fluid solution high-resolution EPR and solid state MAS NMR spectroscopy. The spin densities located on the atoms of the molecule could be probed since 1 H, 13 C, 14 N, and 31 P are nuclei active in NMR and EPR, and lead to a precise spin distribution map for the triradical. The experimental investigations were completed by a DFT computational study.…”

“…[8][9][10][11][12][13] The information gathered for a series of nitronyl nitroxide substituted phosphine derivatives established the presence of spin density at phosphorus and, more important, a spin distribution in accordance with high-spin ground states for the polyradical derivatives. [14] Unfortunately, too weak intramolecular exchange interactions due to the poor spin delocalization for nitronyl nitroxide radicals hampered the observation of the actual magnetic behavior exhibited by the molecules.…”

“…This has been confirmed by the spin distribution mapping undertaken by fluid solution high-resolution EPR [4] and NMR [1] spectroscopy which have proven to be very efficient tools in assessing the magnitude and sign of the spin density borne by the atoms of organic open-shell molecules. The spin densities located on all atoms of the molecule could be probed since all are active nuclei in NMR and EPR, that is, 1 H, 13 C, 14 N, and 31 P, and lead to a precise spin distribution map. These experimental investigations were completed by a DFT computational study.…”